We’ve already seen contrarotating props on the 4-prop OpenPPG electric, so your question really must be about putting them on the same axis in order that they might be larger relative to the standard circular cage size. I don’t see where “acceptance” enters into consideration. Practicality and geometry will determine acceptability. Greater depth is required to place two or more props on the same axis, which on a PPG places more backward counterlever force on the pilot when trying to stand before starting or regain footing upon landing. Electric motors can be made fairly flat, and the props can be mounted in the same plane with them given a mounting-ring structure or mounting of individual blades to an external rotor housing around a stator core, though certain minimal clearances must be maintained between the planes of each rotating prop. Nonetheless, one would have to expect the overall minimum depth of the prop assembly to be double what is experienced currently with PPG props. Would the elimination of the existing minor P-force roll/yaw effect during acceleration be sufficient justification for the increased complexity and weight/balance effects? Might battery packs need to be worn on the pilot’s chest to offset the weight on his back? Who knows? Maybe that could provide a convenient breastplate mounting point for a tablet or smartphone providing flight instrumentation, and maybe it could provide the pilot with a bit of warmth to offset the colder temperatures at altitude.
Co-axial mounted motors seem to be very popular in heavy lift applications (cars/drones). 4 motors in a more compact co-ax configuration is interesting to me and I gave it some consideration, there were just too many unknows for a first paramotor build.
Because the motor structure assembly could take up significantly less X-Y space, maybe the packs could still go behind the pilot below the motor structure. The screen shot below shows 4 M-10s with 22" props and 4 Bonkas…just for fun.
Voltair – What is your battery-pack voltage when fully-charged? What is the voltage required for your motors? How long is your flight endurance, generally? Are you using a Bonka pack? What would be required to switch to a Foxtech Diamond Li-ion 33Ah pack of appropriate voltage, for the sake of extending flight duration? I’m asking you because I don’t find such details or options as alternative battery packs, nor even the battery and motor voltage specs, discussed on the OpenPPG X4 home (sales) page.
I can definitely see how a set of two heavy wooden props would create a really bad throttle response. I think It would be interesting to redo this now that we have E-props that are very light weight and we have better capability’s at making narrow props with a low CF ratio that may allow us to use full size 130cm props for far better efficiency. (they used dual 90cm)
But electric motors with their linear torque curve should be better at avoiding any “throttle lag”. When you drive an electric vehicle it’s much more able to lurch forward, as compared to a gas-powered vehicle, because of that instantaneous torque which does not have to ramp up.
As for added depth for coaxial, just make the 2nd propeller a slightly smaller diameter than the 1st one, to avoid prop contact at lesser axial separation distance.
Also, give ithat 2nd prop slightly more prop twist, and run it at a slightly slower rpm.
These things would provide more efficiency gains without as much gains in noise.
You say more weight, but coaxial contra-rotating should enable reduced prop diameter overall, which can reduce weight and also help on noise.
I said that the wood props that were being used weighed a lot. He was using a 90 cm props(~36 inch). One of the most common 36¨ wood props is made by xoar and it weighs 496 grams and he had two of them for nearly 1kg in props. On the other hand a single 130cm (51 inch) carbon E-prop for the Top 80 engine that he was using only weighs 460 grams. So even if he went with two 130 cm E-props he could still save weight.
One big misconception that many people have with counter rotating props is that you could replace say a 100 inch prop with counter rotating 70 inch props or do something like what chrismichaelwhuuuut drew up. Sadly this is not how it works. Its sort of like the difference between two and four blade props, you cant just cut the effective area of the prop in half because you have twice the number of blades. So on may be able to replace a 130 cm prop without 120 cm counter rotating props without much performance loss, but you can´t to go 90 cm props like the person did in the video without major performance loss.
But contra-rotating props have been used on other types of propeller aircraft – surely there must be some performance benefit, or else they’d have never come into use. It’s not just purely an anti-torque thing, right? There’s an improvement in the thrust efficiency.
Assuming you are referring to coaxial propellers, each prop creates pressure waves. My guess is that the noise is coming from the pressure waves of the first prop colliding with the second prop. It’s a somewhat comparable effect that creates the sound in air raid sirens, but the sirens exploite it to be louder and typically a higher pitch.